This invention relates to an optical scanning system for a copying device, and, more particularly to a system utilizing four optical elements, including a scanning lens, which moves at four different speeds relative to each other.
Various optical systems known in the art achieve a certain degree of compactness by utilizing a full rate-half rate scanning mirror pair which is mounted for parallel movement beneath a document to be copied. U.S. Pat. Nos. 4,113,373 and 3,832,057 and the Xerox "3100" Copier disclose scanning systems representative of this technique. In this type of system, two components, the full-rate and the half-rate mirrors are moving at the predetermined relationships. The projection lens and, typically, a photoreceptor mirror, are held fixed during a scanning mode. For these prior art optical systems, a reduction capability is imparted by moving the projection lens towards the photoreceptor and adjusting the position of the photoreceptor mirror to maintain the required total conjugate. These movements, however, are initiated upon selection of the reduction mode and do not take place during the scanning operation. U.S. Pat. No. 4,095,880 discloses such a scanning system illustrating a scanning mode of operation in a reduction mode.
For scanning systems of the type disclosed above, total conjugates typically fall within the 28"-35" range. The Xerox "3100" copier for example, has a 29.9 inch total conjugate. It would be very desirable to reduce this conjugate length even further since a shorter conjugate length reduces the dimension of the machine housing needed to enclose the optical system which, in turn, results in reduction in overall machine size. The advantage of more compact copier designs are well appreciated in the art. They include savings in material and construction costs and greater customer acceptance because of reduced space requirements and increased portability.
The present invention is therefore directed to a novel scanning system which reduces conventional conjugate requirements by half. This reduction is achieved by adding motion to the projection lens during the scanning mode, permitting the object-to-lens and lens-to-image plane distances to be significantly reduced. In order to compensate for the lens movement, two additional moving optical components are introduced into the scanning system resulting in a multiple (four) rate document scanning system. More particularly, the invention relates to a multi-rate scanning system for scanning a document lying in an object plane and projecting an image along an optical path onto a photoreceptor plane, said system including the following optical elements: an illumination/mirror scanning assembly including a first mirror associated with the illumination means and adapted for movement in the scan direction at a velocity V1 and a second mirror adapted for movement in the scan direction at a velocity V2; a projection lens lying in a plane parallel to said illumination/mirrror assembly and adapted to move in the scan direction at a velocity V3; and a third mirror means adapted to move in the scan direction at a velocity V4; and a drive arrangement for driving said optical elements at a velocity relatively whereby V1&gt;V2&gt;V3&gt;V4.